An x Digital Subscriber Loop (x digital subscriber Loop, xDSL) technology includes: an Asymmetrical Digital Subscriber Loop (asymmetrical digital subscriber Loop, ADSL) technology, and a Very-high-speed Digital Subscriber line2 (very-high-speed digital subscriber line2, VDSL2) technology. The VDSL2 technology is applied in a broadband access device more and more widely.
A line driver of an xDSL subscriber board, for example, a VDSL2 subscriber board, is generally a class AB line driver (also called a CLASS AB line driver). A power supply voltage (V+ and V−) of the class AB line driver generally needs to satisfy a maximum voltage swing requirement of an output signal. However, in most cases, the output signal of the VDSL2 subscriber board is a small voltage swing signal, and a proportion of large voltage swing signals is small, so that currently a problem of low power output efficiency exists in the line driver of the VDSL2 subscriber line board.
In order to improve the power output efficiency of the line driver of the xDSL subscriber board, for example, the VDSL2 subscriber board, a currently-adopted method includes: adopting a class H line driver (also called a CLASS H line driver) as the line driver of the VDSL2 subscriber board, where the class H line driver includes the class AB line driver and a signal detection and charge pump circuit. The signal detection and charge pump circuit is configured to detect a voltage swing of an input signal. When the voltage swing of the input signal exceeds a predetermined threshold level, the signal detection and charge pump circuit linearly adjusts a power supply voltage of the class AB line driver according to a signal waveform of an excessive part, so that the power supply voltage of the class AB line driver can follow a voltage waveform of an output signal. In this way, when a subscriber port of the VDSL2 subscriber board outputs a small voltage swing signal, the power supply voltage of the class AB line driver is of a small voltage value, thus improving the power output efficiency of the class AB line driver and reducing energy consumption of the VDSL2 subscriber board.
In the process of implementing the present invention, the inventor finds that: The subscriber port of the xDSL subscriber board, for example, the VDSL2 subscriber board, generally supports a various working modes (for example, working modes such as 12a, 17a, 8b, and ADSL2+ specified in the ITU standards), and a big difference exists between maximum transmission power of signals on the subscriber port in different working modes and a big difference exists between voltage peak-to-average ratios of signals on the subscriber port in different working modes, but the power supply voltage provided for the class H line driver and the predetermined threshold level are fixed, so that the subscriber port of the xDSL subscriber board, for example, the VDSL2 subscriber board, cannot achieve optimal power output efficiency in various working modes. Furthermore, static power consumption of the class H line driver including the signal detection and charge pump circuit is higher than that of the class AB line driver. In this way, when the subscriber port outputs a small power signal, the power consumption of the class H line driver is higher than that of the class AB line driver.